1. Field of the Invention
The present invention relates to compositions and methods for treating, drilling, cementing, and casing boreholes in subterranean formations, particularly hydrocarbon bearing formations, and to drilling fluids for use in such drilling operations. More particularly, the present invention relates to oil and synthetic fluid based drilling fluids comprising invert emulsions, and drilling fluid additives that enhance suspension characteristics of such drilling fluids, for drilling, and for transport.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Most drilling fluids or muds are water-based. However, oil or synthetic fluid-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”)) holes, but may be used in other holes penetrating a subterranean formation as well. Unless indicated otherwise, the terms “oil mud” or “oil-based mud or drilling fluid” shall be understood to include synthetic oils or other synthetic fluids as well as natural or traditional oils, and such oils shall be understood to comprise invert emulsions as well as all oil systems.
A key parameter to drilling fluid performance is the ability of the fluid to gel upon standing. This property is very important, as the gel formed must form quickly enough and be strong enough to adequately suspend drill solids and weighting materials in the well-bore annulus while not being so strong as to be unbreakable or to cause excessive differences in the drilling fluid surface density and equivalent circulating density (ECD) downhole when circulation of the drilling fluid is restored.
Clay-containing, water-based drilling fluids typically gel satisfactorily. However, most oil based drilling fluids exhibit gel structures that are either “flat” or too strong. A “flat” gel does not build substantial strength over time. A gel that is too strong builds progressive gel strength over time, but the gel is harder to break when fluid flow is restored. Further, when these harder-to-break gels are finally broken, they typically shear thin back slowly to their previous viscosity, rather than quickly returning to a shear thinned rheology commonly seen with water-based drilling fluids or even the “flat” gel (oil based) fluids.
An exception to these generalities are the “fragile gel” drilling fluids taught in U.S. Pat. No. 6,887,832 B2, filed Jun. 19, 2002 and issued May 3, 2005, of Jeff Kirsner, et al. and assigned to Halliburton Energy Services, Inc., Duncan, Okla., the entire content of which is incorporated herein by reference, and U.S. patent application Ser. No. 10/292,124, filed Nov. 12, 2002, of Jeff Kirsner, et al. and assigned to Halliburton Energy Services, Inc., the entire content of which is incorporated herein by reference. The fluids of that patent and patent application are oil (or synthetic fluid)-based, and yet quickly build a gel type structure on standing that is easy to break when fluid flow is restarted, closely mimicking the behavior of water-based drilling fluids. There remains, however, a desire to be able to impart fragile gel behavior or fragile progressive gel structure to a broader range of oil-based drilling fluids than the fluids disclosed in that patent and patent application.
The ability of a drilling fluid to suspend drill solids and weighting materials has commonly been associated with the viscosity of the fluid, although the concepts of suspension ability and viscosity have differences. Nevertheless, increasing the fluid viscosity is often seen as an effective means of suspending weighting materials and drill solids in the fluid. Such increase is commonly effected by adding organo-clays or viscosifying polymers and surfactants to the fluid. Increasing viscosity, however, can negatively impact fluid rheology, risking making the fluid too thick for optimal drilling conditions. Also, organo-clays add solid particles to the fluid which reduce the amount of drill solids the fluid can tolerate and thereby reduce the useful drilling lifetime of the fluid without treatment. Surfactants can interact with emulsion(s) already present in the drilling fluid, changing the fluid flow properties and rendering fluid performance less predictable.
Suspension of weighting materials in drilling fluid is important not only during drilling, but also during transport of drilling fluid to the drilling site. Oil-based drilling fluids are typically prepared at a mixing plant and transported to a drilling site (also called a rig site), often by boat. Such transport time can range from a few days to several weeks during which time the weighting material in the fluid can settle out of the fluid to the bottom of the transport vessel. Such settling increases expense in cleaning the transport vessel and changes the fluid weights. Currently, the fluid is keep circulating during transport to avoid settling or is rendered very thick with permanent or temporary viscosifiers.
There continues to be a need, and consequent search, for improved ways of enhancing the ability of oil-based drilling fluids to suspend weighing materials and drill cuttings.